The present invention relates to techniques for generating seismic waves in the earth, and more particularly to techniques for generating seismic waves substantially below the surface of the earth, such as in a well borehole.
In the exploration for oil and gas it is desirable to discover as much as possible about the nature of the subsurface structure of the earth in the area of interest. Numerous techniques have been developed over the years to aid in generating such information. A commonly used and highly successful technique is that of seismic exploration wherein various sensors, called geophones, are placed in an array on the earth's surface. A seismic wave of some type is generated in the earth's surface in the vicinity of the geophones which then listen to and record the reflections of the wave from the various substrata in the earth. From such data, geologists and geophysicists are able to determine certain characteristics of the subsurface.
Once actual drilling of a well has begun in a selected site, it is typical to attempt to gather as much data as possible about the subsurface structure from the borehole itself. Accordingly, special instruments are lowered down the borehole from time to time to measure physical parameters such as electrical resistivity. Core samples are taken from the borehole wall from time to time and analyzed. Another useful technique is to place an array of geophones on the surfaace of the earth around the borehole and to generate a seismic wave deep in the borehole. This technique can give valuable additional information about the nature of the structure.
One of the major problems in the past has been techniques for generating such a seismic wave in the borehole. The use of explosives has been tried with limited application because the size of the charge required to generate a seismic wave of sufficient strength may also cause severe damage to the borehole itself. Several other techniques have been proposed such as dropping a heavy weight, usually attached to the drill string, on the bottom of the hole to generate the seismic wave. It has also been known to attach special devices to the end of the drill string which are rotated by the passage of drilling fluid through the drill string and emit a sound wave somewhat analogous to the operation of a siren or whistle.
None of the techniques described have proven to be entirely satisfactory and there remains a longstanding need within the industry for an improved method for generating a strong seismic wave down a borehole.